Extended Nucleation and Superfocusing in Colloidal Semiconductor Nanocrystal Synthesis

The hot injection synthesis of colloidal semiconductor nanocrystals is renowned for producing nanocolloids with superb size dispersions. Burst nucleation and diffusion-controlled size focusing during growth have been invoked to rationalize this characteristic, yet experimental evidence supporting the pertinence of these concepts is scant. Using a well-established CdSe synthesis followed with in situ X-ray scattering, we show that nucleation is an extended event that overlaps with growth and can last for 15-20% of the reaction time. Moreover, we find that size focusing outpaces predictions of diffusion-limited growth. This observation supports the conclusion that nanocrystal growth is dictated by the surface reactivity, which drops sharply for larger nanocrystals. Kinetic reaction simulations demonstrate this so-called superfocusing can lengthen the nucleation period and promote size-focusing. The finding that narrow size dispersions can emerge from the counteracting effects of extended nucleation and reaction-limited size focusing ushers in an evidence-based perspective that turns hot injection into a rational scheme to produce monodisperse nanocolloids.